In recent years, the demand for compound semiconductors especially Group III-V compounds (e.g. GaAs) has been growing because of their being superior in performance charcteristics to the conventinal silicon semiconductors. For the production of such Group III-V compound semiconductors, there are known, among others, the so-called molecular beam epitaxy (MBE) process which comprises causing atoms required for a compound to be epitaxially grown to evaporate from a solid material using a heat gun and causing them to collide, in the molecular beam form, against a substrate in an ultrahigh vacuum to thereby cause growth of a film of said material on said substrate, and the so-called metal organic chemical vapor deposition (MOCVD) process which comprises introducing the vapor of methyl-metal or ethyl-metal compound into a reaction chamber at atmospheric pressure or under reduced pressure by means of a carrier gas such as H.sub.2, allowing said vapor to mix with a Group V metal hydride and allowing the reaction therebetween to take place on a heated substrate for crystal growth.
However, the MBE process among said two processes is not suited for a large-scale production, hence can hardly meet the needs of the market because .circle.1 it requires about 10.sup.-11 Torr of ultra-high vacuum, .circle.2 downtime generates when refilling material and .circle.3 it requires a substrate rotating mechanism in order to conduct a homogeneous growth. Therefore, MOCVD process is now paid attention and practically used. However, it has disadvantages in that .circle.1 a distribution is easily caused in a flow direction and it is difficult to analyze the flow at a scale-up since it is a process in a laminar flow area and .circle.2 reactant gas is expensive and the utilization efficienty of the reactant gas is low because of the growth mechanism. Since a large quantity of unreacted gas, which is toxic, is produced because of the efficiency of reactant gas utilization being low, as mentioned above, since the carrier gas constitutes an additional waste gas portion, a large quantity of a toxic waste gas is discharged, and this fact leads to waste gas disposal problems.
Thus, since each of the MBE process and the MOCVD process have disadvantages respectively, it is desired to provide an apparatus for producing semiconductors removed these disadvantages completely. Accordingly, the inventors succeeded in developing an apparatus for producing semiconductors in which advantages of both MBE and MOCVD process are incorporated and filed a patent application (Japanese patent application No. 63-191060). The structure of this apparatus is shown in FIGS. 5 and 6. In these figures, the reference numeral 101 indicates the vacuum chamber of vacuum chemical epitaxy, the vacuum chamber 101 has a reaction chamber 102 therein, which is formed by a base plate 106, surrounding walls 107 and a top plate 108 placed, slidably in one direction, on the upper edges of the surrounding walls 107. The top plate has, in the middle portion thereof, openings 108a. Disc-form GaAs substrates 113 are detachably mounted on the openings 108a respectively. The surrounding walls of the reaction chamber 102 have exhaust ports 110 at certain given intervals around the same. The total area of these exhaust ports 110 is preferably about 4% of the surface area of the top plate 108 of the reaction chamber 102. The base plate 106 has nozzle openings 109 formed at predetermined intervals therein, which are in communication with openings 109 or 134 in the ceiling of a first dispersing chamber 104 disposed under the reaction chamber 102. Each opening 109 is in communication with the first dispersing chamber 104, whereas each opening 134 is in communication with a second dispersing chamber 124 via a duct 119 which passes through the first dispersing chamber 104. The first dispersing chamber 104 is in communication with a starting material inlet tube 121. Said starting material inlet tube 121 serves for introducing into the first dispersing chamber 104 a Group III compound (reactant gas) such as trimethylgallium (TMGa) or triethylgallium (TEGa). The second dispersing chamber 124 has an opening in the lower part, and an exhaust valve 136, suitably a poppet valve, is disposed displaceably in said opening for opening or closing said opening. Said second dispersing chamber 124 is in communication, through one side wall thereof, with a starting material inlet tube 121. Through said inlet tube 121, an n-type or p-type dopant or a Group III compound such as triethylaluminum (TEA l) enters the second dispersing chamber 124. A feeding tube 142 for feeding a Group V compound such as AsH.sub.3 to the reaction chamber 102 has a plurality of holes 142a and 142b at certain definite intervals and in two rows (right and left). A heater 105 is disposed above the top plate 108 of the reaction chamber 102, with a leveling plate 105c. In this apparatus for MESFET epitaxy layer growth, the reaction chamber 10 is fitted with the substrates 113 (the surfaces face below respectively) thereon, then, the vacuum chamber 101 is evacuated to a vacuum of less than 10.sup.-7 Torr and the heater 105 is electrically loaded so that the heater 105 can generate heat. A Group V compound, such as AsH.sub.3 is fed to the feeding tube 142 with the substrate temperature 500.degree. C., so that it enters the reaction chamber 102 through the holes 142a and 142b. The Group V compound thus fed to the reaction chamber 102 flows toward the exhaust ports 110 across the surfaces of the substrates 113. During the flow, AsH.sub.3 or TEAs is collided against the walls of the reaction chamber, which are hot walls, many times and thermally cracked to give As.sub.2. After the temperature of the substrates reaches predetermined process temperature (600.degree..about.650.degree. C.), a Group III compound such as triethylgallium (TEGa) is supplied into the first dispersing chamber 104 from the starting material inlet tube 121 of the reaction chamber 102, is mixed homogeneously and then is blown toward the substrates 113 from nozzles 109 in a homogeneous molecular density. At this time, since the mean free path of molecules of the Group III compound is set longer than the distance from orifice to wafer, the molecules of the Group III compound reaches substrates without having dispersion by collision between material molecules. The molecule of the Group III compound, together with As.sub.2, come into contact with the surface of the substrates 113 and grows on said surface in the form of an undoped gallium arsenide (GaAs) layer or the like. The unconsumed compound that has not come into contact with the substrates 113 leaves the reaction chamber via the exhaust ports 110 and enters the vacuum chamber 101, which they then leave laterally under the action of an exhaustion means. Then, an n-type dopant, either alone or in admixture with the above-mentioned Group III or V compound, is fed to the reaction chamber 102 from the second dispersing chamber 124 so that an n-type active layer can grow on the surface of said undoped GaAs layer. Thereafter, all the gas supplies are discontinued and the system is maintained as it is for about 15 minutes. Then, the substrates 113 are cooled and then taken out of the reaction chamber 102 (hence, from the vacuum chamber 101). In this way, Group III-V compound semiconductors layer can be obtained.
However, in the apparatus for producing semiconductors with said structure, when growth of substrates with large area or a plurality of substrates is conducted, that is, when the distance between a supplying tube and exhaust ports becomes long, there is a disadvantage in that the distribution of the molecular density of the Group V compound is caused between a supplying tube of Group V compound such as AsH.sub.3 and the exhaust ports, and thereby it is difficult to form an homogeneous semiconductor layer in some case (when the layer grows at low V/III ratio). As shown in FIG. 7, since peripheral part of the substrates 113 are supported by substrate holding part comprising a supporting part 108b along the whole circumference, molecular beams which go upward as an arrow mark from the lower part of the reaction chamber 102 are obstructed by said supporting part 108b and do not reach the peripheral part of the substrates 113. Therefore, the peripheral parts of substrates 113, as shown in FIG. 8, are left as untreated part and are thus uneconomical.
Accordingly, it is an object to provide an apparatus for producing semiconductors which can distribute all the reactant gas into the reaction chamber in a homogeneous state.